Diameter-selective extraction of single-walled carbon nanotubes by interlocking with Cu-tethered square nanobrackets

We have been working with carbon nanotube separation through host-guest chemistry. Herein, a new macrocyclic host molecule, Cu-tethered square nanobrackets, is designed, synthesized and applied to single-walled carbon nanotubes (SWNTs) for their diameter-based separation. The complexation between copper ions and dipyrrin moieties of the nanobracket gives Cu-tethered square nanobrackets, which is confirmed by absorption, Raman and MALDI-TOF mass spectra. Upon extraction of SWNTs with the nanobracket and copper(II), in situ-formed square Cu-nanobrackets are found to interlock SWNTs to disperse them in 2-propanol. The interlocking is confirmed by Raman spectroscopy after thorough washing of the extracted SWNTs. Pristine SWNTs were recovered through demetalation of the interlocked ones along with the nanobracket. Raman and absorption spectroscopies of the extracted SWNTs reveals the diameter enrichment of only several kinds of SWNTs in the diameter range from 0.94 to 1.10 nm among ≈20 kinds of SWNTs from 0.76 to 1.20 nm in their diameter range. The diameter selectivity is supported by the theoretical calculations with the GFN2-xTB method, indicating that the most preferred SWNT diameter for the square Cu-nanobrackets is 1.04 nm.

Interlocking of Single-Walled Carbon Nanotubes with Metal-Tethered Tetragonal Nanobrackets to Enrich a Few Hundredths of a Nanometer Range in Their Diameters

We have separated carbon nanotubes through host-guest complexation using host molecules named "nanotweezers" and "nanocalipers". In this work, a host molecule named tetragonal "M-nanobrackets", consisting of a pair of dipyrrin nanocalipers corresponding to two brackets "[" and "]" tethered by two metals (M), is designed, synthesized, and employed to separate single-walled carbon nanotubes (SWNTs). A facile three-step process including one-pot Suzuki coupling is developed to synthesize M-nanobrackets in a 37% total yield (M = Cu). Upon extraction of SWNTs with a square nanobracket and Cu(II), in situ formed tetragonal M-nanobrackets are found to interlock SWNTs to disperse them in 2-propanol. The interlocking is confirmed by absorption and Raman spectroscopy as well as transmission electron and atomic force microscopy. Especially, Raman spectroscopy is utilized to prove the interlocking of SWNTs; Cu-nanobrackets are found to show inherent resonance Raman signals and affect the SWNT signals, or a radial breathing vibration, due to the rigid rectangular structure of Cu-nanobrackets. The interlocking is facilely and thoroughly released through demetalation to recover the pristine SWNTs as well as the square nanobracket. Such chemically controlled locking and unlocking for SWNTs are one of the characteristics of our separation process. This enables a precise evaluation by Raman, photoluminescence, and absorption spectroscopy of the diameter selectivity to SWNTs, revealing the diameter enrichment of only three kinds of SWNTs, (7,6), (9,4), and (8,5), in the 0.02 nm diameter range from 0.90 to 0.92 nm among ∼20 kinds of SWNTs from 0.76 to 1.17 nm in their diameter range.

Extraction of carbon nanotubes with porphyrin- and pyrene-based nanotweezers and nanocalipers: Insight from the association constants

Strong interaction between porphyrin and nanocarbons including carbon nanotubes (CNTs) has been utilized to form stable complexes between them. We have applied the stable complex of CNTs with V- and U-shaped porphyrins and pyrenes, designated as nanotweezers and nanocalipers respectively, to separation of CNTs through their extraction to liquid phase. In order to know more about the CNT complex in liquid phase, we determined the association constants and the extraction ability of the host molecules, porphyrins, pyrenes, porphyrin- and pyrene-based nanotweezers and nanocalipers, to single-walled carbon nanotubes (SWNTs). As we expected, the order of the association constant is nanocalipers > nanotweezers > monoporphyrins > monopyrenes in the host molecules. The extraction ability of these host molecules is roughly proportional to the order of the association constants, but mismatching of the cavity size of the host and the diameter of CNTs is considered to become more apparant in liquid phase than in solid phase, destabilizing the complex in liquid phase and lowering the extraction ability.

Synthesis of flexible nanotweezers with various metals and their application in carbon nanotube extraction

Flexible nanotweezers including Co(ii) dispersed single-walled carbon nanotubes in methanol.